The pineal gland of the mouse, Peromyscus leucopus, is believed to be particularly important in short day-induced adjustments in reproduction and temperature regulation. Chronic or daily administration of melatonin, a pineal product, elicits numerous responses which mimic the effects of a winter photoperiod. Recent studies have demonstrated that the putative brain site(s) for melatonin action are in the anterior hypothalamus. Both chronic implants and daily injections of melatonin in this area cause responses which mimic the effects of a winter photoperiod. This study will precisely localize these sites of melatonin action through the use of a stereotaxically implanted cannula containing a melatonin-beeswax mixture. A unique feature of this cannula is that a uniform and predetermined melatonin release rate can be maintained throughout the study. This will permit adjustment of the melatonin dose so that a relative threshold for each brain site can be determined. This cannula will also be used to provide acute melatonin doses at specific times during a 24 h. period. A second aspect of this study will isolate and characterize brain melatonin receptors in P. leucopus. Daily change in receptor number and affinity will be determined in mice exposed to a short and long day photoperiod. Preliminary receptor studies will be completed in the laboratory of Dr. Lennard Niles (McMaster University Medical Centre, Hamilton, Ontario) who has experience working with melatonin receptors. The health relevance of this study is that it will provide new information on how hormones act on the brain to regulate multiple adjustments associated with mammalian reproduction and temperature regulation.